This slide shows the balance of forces on a descending
Wright 1902
glider.
The flight path of the glider is along the thin black line, which
falls to the left. The flight path intersects the horizontal, thin, red line at an
angle "a" called
the glide angle.

The
tangent
of the glide angle, tan(a), is equal to the vertical height (h) which the
aircraft descends divided by the horizontal distance (d) which the aircraft flies across
the ground.

tan(a) = h / d

The tangent of the glide angle is also related to the
ratio
of the
drag, D,
of the the aircraft to the
lift, L.

What good is all this for aircraft design?
If we combine the two equations into a single equation through the
tan(a), and invert the equation, we get:

L / D = cl / cd = d / h = 1 / tan(a)

The lift divided by
drag is called the L/D ratio, pronounced "L over D ratio."
From the last equation we see that
the higher the L/D, the lower the glide angle, and the greater the distance that
a glider can travel across the ground for a given change in height.
Because lift and drag are both
aerodynamic forces,
we can think of the L/D ratio as an aerodynamic efficiency factor
for the aircraft.
Designers of gliders
and designers of cruising aircraft want a high L/D ratio to maximize the
distance which an aircraft can fly.
It is not enough to just design an aircraft to produce enough lift to
overcome weight. The designer must also keep the L/D ratio high to
maximize the range of the aircraft.

We could have used D/L as the efficiency factor, but then the lower the
factor the better the aircraft. Engineers usually define efficiency factors
so that "bigger" is "better".